Currently, the majority of all vehicles driven today use front-end accessory drive alternators that contain Lundell style rotors, also known as “claw pole” rotors. The rotor provides the alternator's magnetic field and rotates within the machine. The rotor includes a field coil made up of a number of insulated copper wires wrapped around an electrically insulated bobbin. The bobbin surrounds a steel hub, and also insulates the field coil from the steel pole pieces which sandwich the field coil to form north and south poles. The magnetic field is generated whenthe field coil is energized and a current flows through the wires.
It is well known that the magnetic field strength that the rotor provides is proportional to the amount of power the alternator can provide to the vehicle system. The field strength is increased by increasing the size of the coil, or by applying more field current flowing through the windings. However, as the current increases in the field coil, the power dissipation in the form of heat goes up at a rate that is squared due to the governing equation P=I2R, where P equals the power dissipation due to heat, I equals current, and R equals resistance of the coil.
Accordingly, is it important to dissipate as much heat from the hot copper field windings as possible. Preferably, heat is dissipated to the relatively cool steel pole pieces. Furthermore, maximizing the amount of field coil space allows the number of wires wraps (turns) to be increased, providing more magnetic field and hence greater output power.
One type of rotor incorporates the steel core into the pole pieces. Thus, each pole piece includes one half of the steel center hub, and a single face-to-face contact region. Additionally, the bobbin may be injection molded out of a plastic material such as nylon 6—6. The nylon offers flexibility to allow the end cap flaps to be bent over when the poles are assembled onto the bobbin. Unfortunately, this design has its drawbacks. More specifically, the flexibility of the bobbin end caps is due to the material being a generally soft polymer that cannot withstand a lot of contact force created when the bobbin is fit between the pole pieces. Furthermore, injection molding of the bobbin requires a rather large material thickness of the bobbin. The thick material inhibits dissipation of heat, and decreases the space available for the field coil. Thin bobbins cannot be formed because the bobbin mold must fill with molten plastic within a reasonable cycle time.
Accordingly, there exists a need to provide an alternator rotor and bobbin that maximizes the available space for the field coil, increases the dissipation of heat to increase the power density of the alternator, and improves the amount of contact force that can be applied between the field coil and the pole pieces.